1. Field of the Invention
The present invention relates generally to gas turbine engines and, more specifically, to frames therein for supporting bearings and shafts.
2. Discussion of the Background Art
Gas turbine engines include one or more rotor shafts supported by bearings which, in turn, are supported by annular frames. Frames include an annular casing spaced radially outwardly from an annular hub, with a plurality of circumferentially spaced apart struts extending therebetween. The struts may be integrally formed with the casing and hub in a common casting, for example, or may be suitably bolted thereto. In either configuration, the overall frame must have suitable structural rigidity for supporting the rotor shaft to minimize deflections thereof during operation.
The struts have a hollow cross section through which pressurized cooling air passes and is routed into a hub. The pressurized air provides rotor purge for the high pressure and low pressure turbines through holes in the hub. The air also provides cooling for the strut and hub in addition to tubes contained within the struts which service the aft hpt bearing. It is important that the pressurized air within the strut and hub not be lost due to leakage. If leakage occurs, the rotor cavity temperatures will be adversely affected.
One example of a bolted turbine frame assembly is a GE90 turbine center frame (TCF) which has an outer strut end connected to the outer case by eight bolts at each of the twelve strut ends. To minimize relative movement between the case and strut end, a shear bolt is used at each location which bounds off the hole in the case and strut end. To assure concentricity between the case hole and strut hole during manufacture, each strut is located relative to the case and each hole is machined through the case and strut in a single pass. The struts are then separated from the case and each previously machined through hole is used as a pilot to machine a counterbore feature for subsequent thread tapping and insert installation. It is desirable to be able to machine the counterbore from a more accessible side of the strut which results in a more producible, reproducible, and less costly design and manufacture of the turbine frame.
An annular turbine frame includes a first ring such as a radially outer casing disposed coaxially about an axial centerline axis and having a plurality of circumferentially spaced apart first ports. A plurality of circumferentially spaced apart hollow struts are joined radially to the first ring by a corresponding plurality of collars. Each strut has radially opposite first and second ends, and a through channel extending therebetween. Each of the collars is disposed between a respective one of the strut first ends and the first ring in alignment with a respective one of the first ports for removably joining the struts to the first ring.
Each of the collars includes a base disposed against the first ring and has a plurality of mounting holes for receiving mounting bolts therethrough to removably join the base to the first ring. The base has a central aperture aligned with the first port. Each collar mounting hole has a hole counterbore though a radially outer portion of the collar mounting hole. A radially inner portion of the collar mounting hole is threaded to receive and hold a threaded insert which includes inner and outer threaded surfaces. A washer is disposed in the hole counterbore and the mounting bolts are disposed through first ring holes disposed through the first ring, the washer, and screwed into the inner threaded surface of the insert.